The present invention relates to the field of metal detectors and in particular, discloses improved forms of metal detectors.
Metal detectors currently work by generating a low intensity magnetic field from a current flowing through a coil of wire (hereafter called the transmit coil) and examining the various types of distortions produced in this field by a surrounding matrix of materials. The distorted magnetic field is generally sensed by a second coil of wire (hereafter the receive coil), via the process of electromotive induction. The receive and transmit coils may also be the same physical coil of wire.
The magnetic field generated by prior known instruments is generally of two kinds (1) a sinusiodially time varying field or (2) a pulsed field. The former is generally known in the industry as a Very Low Frequency (VLF) or Transmit/Receive (TR) type, while the latter is generally known as a Transient ElectroMagnetic (TEM) or Pulse Induction (PI) type.
It is well known that a flowing current generates a magnetic field such that the field strength, B, is proportional to the current, I. Thus a voltage applied across a coil of wire will generate a magnetic field If this voltage is a time varying one, then the magnetic field is also a time varying field. Second, a voltage is generated in a coil of wire due to a change in the magnetic flux through the coil, xcex5=dxcfx86/dt. For a coil of fixed geometry this implies xcex5=k*dB/dt; where k is a constant.
Thus, in conductive materials, a changing magnetic field will induce currents. These currents are referred to as xe2x80x9ceddy currentsxe2x80x9d.
For magnetic materials this process is generally not relevant to the present application due to the high resistance of these materials. Instead, other processes are relevant. In general, when a magnetic field is applied to a material, the magnetic flux density will not be that solely due to the applied magnetic field. For diamagnetic materials, the flux density is reduced, for paramagnetic materials the flux density is increased, and for ferromagnetic materials, the flux density is increased substantially. For the purposes of this application, xe2x80x9cFerromagneticxe2x80x9d refers to the broad class of materials that include ferromagnetic, antiferromagnetic and ferrimagnetic materials. Due to the finite response time of the processes involved, the changes in flux density will not generally be in phase with the applied field, but will lag slightly. In general, the dominant effect is due to the ferromagnetic properties of materials.
In most VLF metal detectors, a sinusiodially varying magnetic field is generated. This will induce eddy currents in conductive materials and changes in the domain alignments of ferromagnetic materials. The changing magnetic field produce by these then induces a voltage in the receive coil. The resultant phase and amplitude shifts of the magnetic field generated by these processes in the surrounding matrix of materials are analysed to provide an indication of the presence of conductive or ferromagnetic materials.
For the purposes of this application, the inherent time decay of the magnetic field due to the finite inductance and capacitance of the coils can be neglected. Also, the inherent finite response time of the electronics can also be neglected. Also, the offset voltages due to the electronics and inherent coil decay time can be ignored. All these effects may be compensated for in the electronics by means of appropriate high pass filters, voltage offsets, or subtractive, techniques as is well known in the art. Furthermore, external EM influence, whether generated by natural or man-made forces, may be removed with the same techniques, as is well known in the art. It is understood that reference throughout the rest of the text to measurements of the received signal implicitly include the possible use of these techniques where appropriate.
In the simplest versions of this type of machine the decay of the voltage in the receive coil is measured at a single time after the end of the transmit pulse, generally some tens of micro seconds, and high pass filtered. The metal detector transmit and receive coils are swept over the ground and any transient signals due to the presence of conductive materials or to ferromagnetic material produces an output signal. These devices are generally limited to searches over areas that contain little or no ferromagnetic containing ground such as beach sand.
In more sophisticated TEM machines, the voltage in the receive coil is measured at two or more times after the end of the transmit pulse and the characteristic time decay of the receive signal (usually the ground) is determined, usually by computing a ratio between the signal strength at two different times, or some additive linear combination of the two voltages. This provides a method of nulling the ambient surrounding matrix of materials, usually ground containing ferromagnetic material. Any variations in the characteristic decay time (as opposed to the overall signal strength) indicates the presence of other material, such as conductive material like a coin or a gold nugget, or ferromagnetic material such as rusted iron artefacts. British patents GB 2,071,327 and GB 2,071,532 describe inventions of this type.
The user of a metal detector often encounters severe problems when using a prior art device in the field due to the presence of ferromagnetic materials that are a natural constituent of the ground. The materials produce distortions in the transmitted magnetic field that vary spatially over a small scale and in an irregular fashion. This results in a severe loss of time to the operator due to continually having to readjust the machine to null out the signal, or in attempting to locate a possible target that does not actually exist. Additionally, the sensitivity of such a machine may need to be reduced due to the inherent variation in ground response. The variable ground response acts as a type of noise.
VLF type detectors are particularly susceptible to such variations in ground response. This is a significant problem when prospecting for gold, which often occurs in areas with a high ground ferrite concentration. There is the additional problem in such searches that man made ferrite and Fe-containing objects such as nails, screws, and cans (which may or may not be rusted) can be difficult to exclude, resulting in further timewasting.
TEM metal detectors have a significant advantage in highly mineralised ground, as the relative variation of the parameter measured (decay time), is significantly less than that for the VLF type (phase shift). Thus the need to continually null the response of the ground is reduced. Attempts to reduce this further by using pulses of different length have been described in Australian patents AU-B-52364/90 and AU-A-31126/93. However, many conductive targets have characteristic decay times that are similar to that of the ground materials. Consequently, nulling out the ground may significantly reduce, the sensitivity to these conductive objects. More importantly, the device is still highly sensitive to rusted and iron containing . objects. Furthermore, this technology is susceptible to electromagnetic interference, such as that generated by electric motors This makes operation in populated areas difficult.
It is an object of the present invention to provide for improved form of method and apparatus for detecting and discriminating ferrous and non-ferrous materials embedded within a matrix of material, for example, the ground.
In accordance with a first aspect of the present invention, there is provided, in a transient electromagnetic or pulse induction type metal detecting apparatus having a receiving coil attached to a sensor, a method of determining a null point comprising determining a series of post transient output values for the sensor at predetermined times; forming a summation of the output values or their negatives; and altering the predetermined times so as to minimise the summation.
At least two of the predetermined times can be identical and preferably the summation of the coefficients of the numbers utilised in the summation is substantially zero.
In accordance with a second aspect of the present invention, there is provided, in a transient electromagnetic or pulse induction type metal detecting apparatus having a receiving coil attached to a sensor, a method of distinguishing between ferromagnetic and conductive materials comprising the steps of monitoring the decay curve of the response of a transient pulse; comparing the decay curve to an exponential decay curve or a power law decay curve to determined a best fit; and utilising the best fit as a measure of the likelihood of detection of ferromagnetic or conductive materials by the apparatus.
In accordance with a further aspect of the present invention, there is provided, in a transient electromagnetic or pulse induction type metal detecting apparatus having a receiving coil attached to a sensor, a method of distinguishing between ferromagnetic and conductive materials comprising the steps of utilising a series of variable length transient pulses; monitoring the decay curve of the response to each of the transient pulses; comparing the decay curve of different length transient pulses; and utilising the comparison as an indicator of the type of material detected by the apparatus.
In accordance with a further aspect of the present invention, there is provided an apparatus for detecting a metal or mineral object in a surrounding medium with a reduced sensitivity to external electromagnetic interference comprising: a transmission search coil for transmitting a magnetic field; a transmission apparatus for energising the transmission search coil with a substantially rectangular voltage producing a pulsed waveform; a receive coil for detecting the changes in the surrounding magnetic field through a voltage induced across the receive coil by changes in the magnetic flux; a measuring means for measuring the voltage across the receive coil at 3 or more selected time intervals after the end of a transmission pulse; and a processing means for processing the voltages and altering the timing intervals such that a null summation is produced for a target medium.
The null summation can be produced by altering the measurement time intervals and linearly combining the voltages across the receive coil at the time intervals such that the coefficients sum to zero, and producing a ground balanced null signal substantially satisfying the following equations 0=xcexa3kiVi; xcexa3ki=0 where Vi is the voltage at the ith time interval after the end of the transmission pulse and ki is a multiplicative factor.
The null summation can be produced by comparing the measured received voltages at given sample intervals to a power law and to an exponential law.
In accordance with a further aspect of the present invention, there is provided an apparatus for detecting a metal or mineral object in a surrounding medium comprising: a transmission search coil for transmitting a magnetic field; a transmission apparatus for energising the transmission search coil with a substantially rectangular voltage waveform producing a pulsed waveform that has pulses of two different, alternating, widths; a receive coil for detecting the changes in the surrounding magnetic field through a voltage induced across the coil by changes in the magnetic flux a measuring means for measuring the, voltage across the receive coil at a selected time interval after the end of a transmission pulse, the time interval being alterable; a processing means of processing the the voltages, and altering the transmission pulse width, and altering the receive timing interval such that a null summation is produced for a target medium.
The null summation can be produced by linearly combining the receive voltages determined after each pulse such that a ground balance null can be substantially achieved according to the equation 0=V11+kV12 where V11 is the voltage measured after the short transmission pulse and V12 is the voltage measured after the long transmission pulse, and k is a multiplicative factor.
In accordance with a further aspect of the present invention, there is provided an apparatus for detecting a metal or mineral object in a surrounding medium with reduced sensitivity to ferromagnetic materials comprising: a transmission search coil for transmitting a magnetic field; a transmission apparatus for energising the transmission search coil with a substantially rectangular voltage waveform producing a pulsed waveform that has pulses of two different, alternating, widths; a receive coil for detecting the changes in the surrounding magnetic field through a voltage induced across the receive coil by changes in the magnetic flux; measuring means for measuring the voltage across the receive coil at two selected time intervals after the end of a transmission pulse, the time intervals being alterable; processing means for processing the voltages, altering the transmission pulse width, and altering the receive timing interval such that a null summation is substantially produced for a target medium.
The null summation can be achieved by linearly combining the receive voltages derived from the short and long transmission pulses satisfying the following equation to produce a ground balanced null: 0=xcexa3ij kijVij where Vi1 is the ith measurement interval after the end of the shorter transmission pulse, where Vi2 is the ith measurement interval after the end of the longer transmission pulse, and kij is a multiplicative factor.
In accordance with a further aspect of the present invention, there is provided an apparatus for detecting a metal or mineral object in a surrounding medium with reduced sensitivity to ferromagnetic materials comprising: a transmission coil for transmitting a magnetic field; a transmission apparatus for energising the search coil with a substantially rectangular voltage producing a pulsed waveform that has pulses of at least three different, alternating, widths; a receive coil for detecting the changes in the surrounding magnetic field through a voltage induced across the receive coil by changes in the magnetic flux; measurement means for measuring the voltage across the receive coil at two selected time intervals after the end of a transmission pulse, the time intervals being alterable; and processing means for processing the voltages, altering the transmission pulse width, and altering the receive timing interval such that a null summation is produced for a target medium.
The null summation can be achieved by combining the voltages derived after each different transmission pulse width such that a ratio of values derived from the receive voltages due to any two of the transmission pulse width types are preferably used to compute expected receive voltages due to the third transmission pulse type and are preferably compared with measured voltages.
The null summation can be achieved as followsxe2x80x94given ti is the ith time interval after the transmission pulse, Vij is the ith measured voltage (at time interval ti) after the jth transmission pulse type, calculating:
V11/V21=(t1/t2)xe2x88x92xcex11
V12/V22=(t1/t2)xcex12
and xcex1i is the spectral index for transmission pulse type i, which depends on the material being interrogated, the null summation, having little sensitivity to ferromagnetic materials being formed as follows:
0=V13+(t1/t2)xe2x88x92xcex13V23